Here's a short explanation of habituation done by a student of Raising Canine's Professional Dog Trainer course – good job, Kathy!
Habituation is achieved by repeatedly evoking a given reflex response, resulting in the reduction in intensity/probability of that response. It is a form of desensitization but the subject is no longer aware of the stimulus, whereas with desensitization, a response is still elicited but the emotion associated with the stimulus is different. Habituation gives us the ability to filter out irrelevant stimuli.
Short term habituation happens after a few repetitions of the stimulus that occur close together. We are no longer aware of the stimulus even though the stimulus is present. For example, a student studying with music on. In the beginning the student sings along with the music. After a while, the student only sings along occasionally. Eventually the student will no longer notice the music – it will become “white noise” in the background. But if the student should leave the room, then return, the student will again notice the music.
Long term habituation happens after many repetitions of a stimulus over a longer period of time, with those repetitions being spaced farther apart in time. We are no longer aware of stimulus even when the stimulus is present, is not longer present, and then returns (or, alternatively, the animal leaves and then returns to the location of the stimulus). An example would be a dog that grew up in a kennel situation. When the dog is brought to a “home” environment in a busy, urban area, it may jump up and run to the window or bark every time it hears cars drive by. After a while, it may pick up its head or twitch an ear toward the sound when cars drive by. Eventually, it will no longer react to cars driving by.
Habituation is important to survival because if the organism were to constantly react to every stimulus, it would not be able to do anything productive and might, in fact, be rendered helpless in the face of danger. Organisms need to be able to automatically (without conscious thought) filter out what is “imminent danger” vs. “need to be aware of” vs. “not dangerous, can ignore.”